Columbia EESC V1003 - General Atmospheric Circulation

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General Atmospheric CirculationTake away Concepts and IdeasWhere we left off on Wednesday…Why is this important? Heat transport by the atmosphere and ocean!Pressure gradient force F (east east) = - (Δp / Δ x) / ρ F (north-south) = - (Δ p / Δ y) / ρThink of pressure fields like a topo mapPressure gradient force …. If there was no earth rotation F is proportional to Δp / Δy and Δp / ΔyNow, enter the Coriolis effectCoriolis Force Cons. of Angular MomentumGeostrophic BalancePressure gradient force …. WITH earth rotation Geostrophic CirculationSurface Pressure… and the winds Strong winds where ∆P is greatestApplication to EarthTrade Winds and the Intertropical Convergence Zone (ITCZ)Hadley Cells Tropical-SubtropicalTropical-Subtropical vegetation patternsGlobal VegetationCirculation CellsPowerPoint PresentationSlide 21Jet Stream Upper level air flowWhy is there a Jet Stream?Jet StreamMonsoonal climateLargest monsoon regions: S. Asia & N. AfricaCyclones (Hurricanes and Typhoons)Birthplace of Atlantic HurricanesHow to make a hurricaneHurricane basics - Forms over warm waters - Cyclonic flow around deep low - Alternating bands of convection/subsidence - Water condensation important energy source - Loses power over landKatrina (Aug. 25, 2005)Slide 32General Atmospheric CirculationTake away Concepts and IdeasGlobal circulation:The mean meridional (N-S) circulationTrade winds and westerliesThe Jet Stream Earth’s climate zonesMonsoonal climateHurricanesWhere we left off on Wednesday…Atmospheric motion is driven by uneven heating of the planet.Atmosphere (and ocean) moves excess heat from tropics to polesFlow is determined by balance between pressure gradients and Coriolis forceWhy is this important?Heat transport by the atmosphere and ocean!Pressure gradient forceF (east east) = - (Δp / Δ x) / ρF (north-south) = - (Δ p / Δ y) / ρHigh/low pressure?High/low pressure?High/low pressure?xyThink of pressure fields like a topo map High pressure “wants” to move to low pressure in the most direct way.If the Earth didn’t rotate - air would flow H --> L.HLHLLLHLHLPressure gradient force …. If there was no earth rotationF is proportional to Δp / Δy and Δp / Δy xyHHLLLNow, enter the Coriolis effectArtifact of Earth’s rotationAn apparent force which acts on motion on the earth’s surfaceConservation of angular momentumFcoriolis = 0 on equator, increases polewardCoriolis Force Cons. of Angular MomentumGeostrophic BalanceNORTHERN HEMISPHERE… the pressure gradient force (∆P) is balanced by Coriolis force.Applies to frictionless flow.Pressure gradient force …. WITH earth rotationGeostrophic Circulation HHLLLInteresting link…Surface PressureHLLHhttp://weather.unisys.com/surface/sfc_con.php?image=pr&inv=0&t=cur&expanddiv=hide_bar… and the windsStrong winds where ∆P is greatesthttp://www.wunderground.com/US/Region/US/2xWindSpeed.htmlApplication to EarthGeostrophic flow gives rise to banded circulation zonesTrade Winds and the Intertropical Convergence Zone (ITCZ)ITCZHadley Cells Tropical-SubtropicalTradewindsHadley CellsTropical-Subtropical vegetation patternsRising limb of Hadley Cell (ITCZ) = Equatorial RainforestDescending limb of Hadley Cell = DesertsGlobal VegetationCirculation CellsTradewindsHadley CellMid-latWesterliesPolar EasterliesFerrell CellPolar CellWindZonesMid-Latitude Westerlies Westerlies = blow from west to eastWinds are highly variable, unsteady, particularly in winter (why?)Polar EasterliesEasterlies = blow from east to westWinds are v. cold, more stableJet StreamUpper level air flowVery fastHigh altitudeVariableWhy is there a Jet Stream?EquatorNorth PoleOccurs where warm and cold air meet5 km altitudeJet StreamHadley circulation is very deep in tropical troposphere (15 km) Aloft, the air must return toward poles, carries with it equatorial angular momentum.Northward motion is deflected to right (east) by Coriolis.No friction … very fast!Jet streamJet streamMonsoonal climateSummer(rain season)Winter(dry season)Why does it rain?Why is it dry?Largest monsoon regions: S. Asia & N. AfricaRainy SeasonDry SeasonWinterSummerCyclones (Hurricanes and Typhoons)Develop around deep LOW pressure cellsCyclonic flow = around low pressure (CC in NH)Intensity due to added power from condensationBirthplace of Atlantic HurricanesHow to make a hurricaneHurricane basics- Forms over warm waters- Cyclonic flow around deep low- Alternating bands of convection/subsidence- Water condensation important energy source- Loses power over landKatrina (Aug. 25, 2005)Trade wind belts: In the tropics, on both sides of the equator, lies a wide region where winds blow from east to west (easterlies) with a slight equatorward tilt. This region is named the trade wind belt, because of the steadiness of the air flow here. Intertropical Convergence Zone (ITCZ): The trade winds from the Northern and Southern Hemispheres converge into a narrow belt close to the equator, nowadays generally referred to as the Intertropical Convergence Zone (ITCZ). The convergence of the trade winds results in rising motion of the colliding air masses (to obey the law of mass continuity).Midlatitude westerlies: North and south of the trade wind belt (in the Northern and Southern Hemispheres, respectively) lie regions where winds tend to blow from west to east (westerlies), and are therefore referred to as the westerly wind belts. Here the winds are highly variable and unsteady, especially so during winter. Subtropics: Between the trade wind regions lie the subtropics - regions of divergence and subsidence, where sunny weather with little clouds and no rain prevails. Most of the Earth’s desert regions are found near 20-30° north and south of the equator.Polar easterlies: Poleward from the westerly wind belt, winds with a generally easterly component prevail. The air here is cold, dry and stable, especially during winter, and is accompanied by subsidence from above.Polar front: The convergence zone between polar easterlies and midlatitude westerlies is referred to as the polar front. It separates between the cold (and dry) polar air, and the relatively warm (and more humid) midlatitude air. The polar front can be thought of as the average expression of the transient frontal systems that move along with midlatitude


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Columbia EESC V1003 - General Atmospheric Circulation

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